The invention relates to static converters of the symmetrical self-oscillator type, whose function is to convert a d.c. into an a.c. of chosen frequency and power. In devices of this type, such as the one shown in FIG. 1, the self-oscillator is composed of two switching transistors 1 and 2, mounted with a common emitter. The collector circuit of each of these transistors is connected to the transformer 11 by means of the windings 9 and 10. These two windings, connected to one of the poles of power supply 14, are crossed, successively, by a current of opposite polarity. Secondary windings 3 and 8, inserted into the base-emitter circuits of the transistors, maintain the oscillations. The function of resistors 4 and 7 is to limit the base-emitter current of the transistors. The second function of the resistors 4, 7, respectively, connected to resistors 5 and 6, is to set a divider bridge for biasing correctly the base-emitter circuit of the transistors. Transformer 11 includes a secondary winding 12 which, connected to capacitor 13, constitutes a parallel self-oscillating circuit which determines the frequency of the device. At the terminals of this self-oscillating circuit, the desired a.c. is available. Such a converter allows, from a battery, provides different functions: supplying one unit normally energized from the main power supply, supplying a high voltage, and supplying a current of high frequency.
Such a converter is afflicted with major disadvantages, as set forth below.
Technical disadvantage: in the switching mode, the transistors must, without breakdown, sustain transient surges, which in the blocking state, can easily reach up to five times the voltage of the power supply itself. Thus, for instance, if we consider the rectified main a.c. voltage as the power supply, we dispose of at least 300 v. and consequently, of transitory surges which could reach up to 1,500 v. which is the maximum Vcbo of the best high voltage transistors. Now, by definition, to sustain the Vcbo, we must short-circuit, in the blocking state, the base-emitter path of the transistors. In the aforementioned device, the eventuality of short-circuiting is impossible, because, in case of transient surges of high frequency, the impedance of base windings 3 and 8 is high, this disadvantage is increased by the addition of the supplementary impedance of resistors 4 and 7. Consequently, such an embodiment can only permit the transistors to sustain transient surges which are of very low Vcbo. So, the large field of application that represents the use of the modified main power supply (110 v. 60 Hz or 220 v. 50 Hz) as a d.c. generator, is under those conditions, unavailable, because the expected reliability would be too reduced.
Industrial disadvantage: the fabrication of the transformer of the device implies strict tolerances of manufacturing, because a perfect symmetry of the self-inductance, serial resistor and self-capacity of the four windings must be guaranteed. In addition, such a transformer has ten terminals to be connected in a precise order and this operation increases the difficulty of its industrial fabrication.
Functional disadvantage: the current that this device produces is not perfectly sinusoidal, which makes it unable to energize, for instance, fluorescents lamps. In this application, indeed, if current alternations are not strictly equal in energy, a migration of mercury to one of the ends of the tube may occur. After a short period of operation, this phenomenon results in the blackening of this end. This blackening cannot happen in the case of a direct supply from the power supply, because its sinusoid is perfect; on the other hand, in such devices, even of the best manufacturing quality, the energy of each current alternation is not always perfectly equal. For this reason, such devices have never been manufactured on a large scale, except to supply emergency appliances.